A successive approximation register analog-to-digital converter (SAR ADC) is capable of converting an analog signal into a digital signal. Referring to FIG. 1, the SAR ADC is formed of a comparator, a register and a digital-to-analog converter (DAC), and converts the analog signal into the digital signal based on the following principle: comparing an analog input signal Vi with an intermediate value of a reference signal VR which is an variable value acquired by an output of a D/A converter, determining to increase or decrease the digital signal input to the DAC according to the comparison result such that the intermediate value of the reference signal VR approximates to the analog input signal Vi, and when the intermediate value of the reference signal VR is equal to the analog input signal Vi, the digital signal input to the DAC is the digital signal corresponding to the analog input signal Vi. That is, the analog-to-digital conversion is implemented by means of successive approximation of the DAC output to the input voltage, and the process of successive approximation may be referenced to FIG. 2. As illustrated in FIG. 2, through the first comparison, the analog input signal Vi is greater than the intermediate voltage VR/2, thus the digital value of a most-significant bit is 1, and the output of the DAC is adjusted; and through the second comparison, the analog input signal Vi is less than the intermediate voltage VR/2, thus digital value of a the second-most-significant bit is 0. Comparisons are made analogously until the analog input signal Vi is equal to the intermediate voltage VR/2, and finally a group of digital values are obtained, that is, a digital signal obtained after the analog input signal Vi is converted by the analog-to-digital converter.
The SAR analog-to-digital converter is used as a key component for interfacing between an analog module and a digital module, and is extensively applied to mobile devices, wireless sensors and the like. Due to the size and endurance of the device, the analog-to-digital converter needs to have a small size and low power consumption, such that the analog-to-digital converter can be conveniently integrated into the circuits of various devices.